1. Field of the invention
This invention relates in general to subsea wells, and in particular to a subsea well assembly having inner, outer, and intermediate wellhead housings, with a port and sleeve for exposing the annulus surrounding the inner wellhead housing for cementing.
2. Description of the Prior Art
One type of subsea well installation utilizes an outer or low pressure wellhead housing that locates on the sea floor. The outer wellhead housing is a large tubular member secured to the upper end of an outer string of casing or conductor pipe. The outer casing will extend to a certain depth in the well, and will normally be cemented in place.
Then, the operator lowers a high pressure wellhead housing into the outer wellhead housing. The high pressure wellhead housing is connected to the upper end of a string of casing that extends through the conductor pipe to a greater depth in the well. The operator pumps cement down the inner casing, which flows back up the annulus between the inner casing and the conductor pipe. The cement returns pass through ports located in the outer wellhead housing.
Then, the operator will drill the well to a further depth. The operator lowers a casing hanger which lands in the interior of the high pressure wellhead housing. The casing hanger secures to a string of casing that extends through the inner casing to a greater depth. This inner casing will be cemented in place. Cement returns flow up the annulus surrounding the innermost string of casing and pass through flowby slots located on the exterior of the casing hanger. A seal will then be placed in an annulus space between the casing hanger and high pressure wellhead housing. Subsequently the operator may complete the well with tubing. Or the operator may drill to even greater depths and support additional casing hangers and strings of casing in the high pressure wellhead housing.
One reason for using multiple strings of different diameter in a well is to protect low pressure water zones in the earth's formation. Hydrostatic pressure of the drilling fluid in the well bore would exceed the formation pressure in certain cases if too much open hole portion is exposed. The drilling fluid weight is selected to be sufficiently greater than the pressures in the various earth formations to avoid a blowout. This weight may be excessive for low pressure formations located uphole in the well and not protected by casing. Drilling fluid may flow into the low pressure formations, which is not desired. In many cases, the use of one low pressure wellhead housing, one high pressure wellhead housing, and multiple strings of casing will be sufficient to adequately protect low pressure water zones in the well.
There are geographic areas, however, in which more casing than normal would be required to protect low pressure water zones. Running an additional casing hanger and string of casing may not always be feasible because of the shallow depth of the low pressure water zone. Also, additional casing strings result in the final production string possibly being smaller than desired.
It has been proposed by others to utilize an additional wellhead housing, resulting in an outer, intermediate, and inner wellhead housing. However, running an inner wellhead housing, without modification, creates problems. Wellhead housings do not have flowby slots on the exterior like casing hangers. Wellhead housings do not have packoffs or seals located between them. Consequently, an inner wellhead housing located within an intermediate wellhead housing would not have any passage for cement returns to flow.